1. Field of the Invention
The present invention relates to an inkjet recording system and an inkjet recording method for recording images on various recording media, using a recording head in which a plurality of nozzle arrays are formed, by discharging ink droplets from the nozzles in the nozzle arrays while moving the recording head.
The present invention is applicable to any piece of equipment using a recording medium such as paper, cloth, leather, nonwoven fabric, an overhead projector (OHP) sheet, and even metal. Specifically, present invention is applicable to office equipment such as printers, copying machines, and facsimile machines, as well as industrial production equipment.
2. Description of the Related Art
Office automation (OA) equipment such as personal computers and word processors are now widely spread. Various recording apparatuses and methods have thus been developed to record information which is input by these pieces of equipment on various recording media. In particular, with the improvements in information processing capabilities of such OA equipment, the processed image information tends to be in color. This trend toward color output is progressing even for recording apparatuses which output processed information. Various recording apparatuses capable of recording color images are available according to costs and functions. Some recording apparatuses are inexpensive with relatively simple functions, while others have a large number of functions which enable a user to select a recording speed and image quality depending on a type of images to be recorded or the intended use.
Inkjet recording apparatuses can be low-noise, low running cost, and compact, and can easily record an image in color. The inkjet recording apparatuses are thus widely utilized in printers, copying machines, facsimile machines and the like. Generally, color inkjet recording apparatuses record color images using three color inks, cyan, magenta, and yellow inks, or four color inks, these three inks plus black ink. Conventional inkjet recording apparatuses generally use special paper which has an ink absorbing layer as a recording medium to record color images with excellent color development free from ink bleeding. Currently, improved inks with properties suitable for recording on “plain paper”, which is used in large quantities by printers, copying machines and the like are practically used.
Serial scan type inkjet recording apparatuses employ an inkjet recording head in which nozzle groups corresponding to each of ink colors used in recording are provided to perform color recording using a plurality of color inks. The recording head can discharge the ink from discharge ports constituting the nozzles. The serial scan type inkjet recording apparatuses sequentially record images on the recording medium by alternately repeating an operation of discharging the ink from the discharge ports in the recording head while moving the recording head in a main scanning direction, and an operation of conveying the recording medium in a sub-scanning direction which intersects the main scanning direction. Thus, a lateral configuration recording head is used in which nozzle groups (nozzle groups to be used) corresponding to each of the ink colors used in recording are sequentially laterally arranged along the main scanning direction. The lateral configuration recording head can discharge ink droplets from the respective nozzle groups onto a same raster during a same recording scan.
To realize high-resolution recording to record higher quality images, it is effective to employ a high-density recording head in which recording elements of the recording head, including the nozzles, are more densely integrated for the lateral configuration head in the inkjet recording apparatuses. Nowadays, even high-density recording heads with nozzle arrays of 600 dpi (about 42.3 μm) are produced by employing semiconductor processes.
Moreover, recording heads are produced in which, to arrange the nozzles at an even higher density, a plurality of nozzle arrays corresponding to each ink color is provided in parallel and arranged so that positions of the nozzles in those nozzle arrays are offset by a predetermined amount in the sub-scanning direction. For example, if two nozzle arrays each of which has a nozzle arrangement density of 600 dpi are arranged in parallel such that the positions of the nozzles in those two nozzle arrays are displaced from each other to achieve 1,200 dpi (about 21.2 μm) in the sub-scanning direction, this results in a recording head with a high density of 1,200 dpi.
Another method for recording higher-quality images is to reduce a size of each ink droplet for image recording. To reduce the size of the droplet, it is effective to use a recording head with downsized recording elements, including nozzles, capable of discharging smaller ink droplets. Today, recording heads suitable for high-definition recording which can discharge ink in 4 to 5 pl amounts are produced.
Thus, higher-quality images can be recorded by discharging smaller ink droplets from densely arranged nozzles.
However, when the lateral configuration recording head is used, the ink discharges from the respective nozzles in the plurality of nozzle arrays lined up in the main scanning direction may affect one another. Ink droplets discharged from the nozzles draw in the surrounding air. Thus, when the recording head is moved at a high speed in the main scanning direction simultaneously with the discharge of a large number of ink droplets, an air flow (air current) is generated, which may adversely affect the discharge of the ink.
A mechanism of generation of an air current will be described in more detail. First, with reference to FIG. 1, the generation of the air current resulting from operation of the recording head will be described.
FIG. 1 illustrates a discharge port forming surface of a recording head H viewed from above. Discharge ports constituting nozzles N are formed on the discharge port forming surface. Nozzle arrays L1 and L2 both discharge ink in a direction orthogonal to a sheet surface of FIG. 1 from their nozzles N. The recording head H records by discharging ink from the nozzles N in the nozzle arrays L1 and L2 while moving in the main scanning direction illustrated by an arrow X in FIG. 1. At this stage, ink droplets discharged vertically below the nozzles N in the nozzle array L1 draw in the surrounding air to form a “gas wall” that moves as if in the direction of the arrow X. Movement of the “gas wall” in the direction of the arrow X generates the air current flowing into a back of the “gas wall” in directions of arrows A in FIG. 1. The air current flows toward a front of the nozzle array L2, which adversely affects the ink droplets discharged from the nozzles N in the nozzle array L2. This can result in the discharge direction shifting.
FIG. 2 illustrates the recording head H viewed from a side direction and the air current behind the “gas wall”. Ink droplets discharged from the nozzles N in the nozzle arrays L1 and L2 in a direction of arrows B cause a downward air current. The direction of the air current may change near a recording medium W to a rearward direction as illustrated by the arrows A.
FIG. 3 illustrates the recording head H viewed from the front in the main scanning direction. FIG. 3 focuses on the nozzle array L2. In FIG. 3, ink droplets discharged from the nozzles located at an end (end nozzles) of the nozzle array L2 may have their discharge direction bent to the inward of the nozzle array L2 as the ink droplets approach the recording medium W due to the influence of the air current in the direction of the arrows A.
If such bending occurs, the ink droplets discharged from the end nozzles impact the recording medium W at positions that are deviated from the proper impacting positions to the inward of the nozzle array L2. This is recognized as an image defect similar to cases where a shift (bias) occurs in the discharge direction of the ink droplets, or ink droplets are not discharged. The discharge direction of the ink droplets discharged from the end nozzles is bent due to the effects of both the air current flowing behind the “gas wall” as illustrated in FIG. 1 and the air current generated from ink discharge as illustrated in FIG. 2.
Thus, the recording apparatuses which employ the conventional lateral configuration recording head may cause image defects due to air currents resulting from the discharge of ink droplets.
Japanese Patent Application Laid-Open No. 2004-142452 discusses a technology relating to effects of air currents in inkjet recording apparatuses. For a multipass recording system that records an image in a predetermined region by a plurality of scans of a recording head, the document discusses a method for controlling an applied ink amount by considering a relationship between a number of scans (number of passes) and a level of adverse effects of an air current. That is, to avoid the adverse effects from the air current, the applied ink amount is controlled based on the number of passes.
Further, in Japanese Patent Application Laid-Open No. 2004-142452, occurrence of the adverse effects on an image due to the air current is avoided without increasing the number of passes, by limiting recording conditions of ink droplets between nozzle arrays which are especially susceptible to the effects of air currents. Japanese Patent Application Laid-Open No. 2004-142452 discusses ink droplets of same color in different sizes, in which as illustrated in FIG. 18, recording dots are limited and applied to a left below region of a curve 801 between a recording nozzle array 1 (large dots) and a recording nozzle array 2 (small dots).
Further, to respond to a demand for higher speed recording of recent years, a driving frequency of the recording head can be improved. In other words, a moving speed of the recording head is increased in the main scanning direction. In this case, a level of the above-described air current effect changes according to the moving speed of the recording head. For example, when the same number of passes is recorded and the moving speed of the recording head is different, the level of the air current effect on the discharged ink droplets will substantially change. Obviously, the level of the air current effect increases when the recording head moves at a higher speed. As a result, impact precision of the ink on the recording medium may worsen and cause image quality deterioration.
Japanese Patent Application Laid-Open No. 2006-21532 discusses a technique for differentiating an amount of ink applied on a paper surface per unit region of the ink droplets discharged from a plurality of nozzle arrays of the recording head based on the recording speed. In this example, different air current limitation conditions for each recording speed are provided as illustrated in FIG. 10. To realize these conditions, different recording data generation curves for each recording speed are discussed as illustrated in FIGS. 21A to 21C.
Japanese Patent Application Laid-Open Nos. 2004-142452 and 2006-21532 respectively discuss two data generation methods for limiting an air current between same-color, different-amount ink droplets.
<Technique 1>
Japanese Patent Application Laid-Open No. 2004-142452 discusses an index method, in which limitation conditions are satisfied by an index pattern. An original image on a host computer is subjected to necessary color conversion processing (first-stage processing for compressing image data expressed in standard color space into a printer color gamut, second-stage processing for separating the image data compressed into the printer gamut into ink colors, gradation correction, and quantization) to generate recording information which has undergone multi-valued quantization for each of the ink colors (CMYK). In a printer engine, the received multi-valued recording information data for each of the ink colors is converted into ink dot data to be applied on the paper surface. As illustrated in FIG. 20, how large and small dots are used (index pattern) in a 2×2 recording pixels is defined for each level of the quantized multi-valued data so that the index pattern is always positioned in an OK region of FIG. 18. FIG. 19 illustrates a graph obtained by plotting the index patterns of FIG. 20 on FIG. 18. The dashed line 802 in FIG. 19 is a line linking between the respective quantization levels. Thus, actually generated data exists somewhere on the line 802.
<Technique 2>
Japanese Patent Application Laid-Open No. 2006-21532 discusses color separation processing which has another SC and SM for each plane for cyan ink and magenta ink in addition to CMYK in the second-stage (color separation) processing when RGB data is converted into the ink colors after the first-stage processing, as illustrated in the block diagram of FIG. 8. The color separation processing is virtually performed for a total of six colors, CMYKSCSM. Further, in Japanese Patent Application Laid-Open No. 2006-21532, the color separation processing is performed using a three-dimensional table and linear interpolation. C and M are given as the data for the large ink droplets, and SC and SM are given as the data for the small ink droplets. The generation conditions for the C and SC, and the M and SM tables are determined to form the dots within an OK region of a limitation curve of each recording speed by changing the limitation curve as illustrated in FIG. 10 based on the recording speed.
However, the techniques satisfying the above limitation conditions have the below-described problems. First, in the technique 1, in the large dot region of 0 to 1 dot in the OK region, a transition in combination of large and small dots occurs in a region significantly below the limitation conditions based on the air current (the solid line in FIG. 19). It is known that a grainy effect of the image is generally better if the large ink droplets are used after using many small ink droplets. In FIG. 19, when there is one large dot, it is desired to record with about 2.5 dots of the small dots, which is just on the air current control line. However, in the case of the technique 1, because the index pattern can only be defined as an integer of the number of dots, the maximum number of small dots which can be used for one large dot is up to two dots. More specifically, when one large dot is discharged, discharging three small dots would be in a no good (NG) region. Thus, the combination of one large dots and three small dots cannot be selected. As a result, the grainy effect is worse than the maximum value on the air current limitation line of 2.5 dots.
Further, the technique 2 performs the color conversion of the input (R, G, B) values into CMYKSCSM using the three-dimensional table and interpolation processing. In this case, if respective (C, M, Y, K, SC, SM) values corresponding to all of the (R, G, B) values of the input data are stored, the data amount is extremely large. Thus, to avoid this, only the (C, M, Y, K, SC, SM) values on points which predetermined (R, G, B) values are discrete (commonly referred to as “grid points”) are stored, and the other (RGB) values are calculated by interpolating based on the (C, M, Y, K, SC, SM) value of the grid points adjacent thereto. Generally, from a calculation speed perspective, linear operation processing is used for the interpolation calculation.
In the technique 2, by generating the (C, M, Y, K, SC, SM) value for each grid point such that it does not exceed the air current control line during the three-dimensional table generation, the air current effects on the image can be prevented. However, even if the (C, M, Y, K, SC, SM) values on the grid points are on the air current control line illustrated in FIG. 10, at the points other than the grid points determined by the interpolation processing, the (C, M, Y, K, SC, SM) values may exceed the air current control line and lie in the NG region.
This problem is not limited to a linear interpolation calculation. Interpolation algorithm and air current generation mechanism have absolutely no relationship to each other. Thus, there is no interrelationship between the air current control line determined by the results of the air current generation mechanism and the interpolation calculation technique. Accordingly, just because the grid point satisfies the air current control line does not mean that the data determined by the interpolation results satisfies the air current limitation conditions. As a result, a region which satisfies the air current control line may be affected by an air current on the image and a part of the image, which is originally good, may be lost.
The (C, M, Y, K, SC, SM) values of the grid points could be generated so that the points other than the grid points are not affected by the air current while considering the interpolation algorithm. However, to perform this control to generate the grid points while satisfying all three-dimensional directions is very complex and time consuming. Producing such system is laborious, and there is no guarantee that an optimum image for the user will be obtained.
Thus, with the conventional techniques for realizing air current control, there is no guarantee that an optimum image will be formed free from the effects of an air current.